Studies on the mechanisms of action of picrotoxin, quercetin and pregnanolone at the GABAρ1 receptor

1 The mechanisms of action of antagonists of the gamma-aminobutyric acid C (GABA(c)) receptor picrotoxin, quercetin and pregnanolone were studied. 2 Ionic currents (chloride), mediated through human homomeric GABA(rho1) receptors expressed in Xenopus oocytes, were recorded by two-electrode voltage clamp. 3 Dose-response (D-R) curves and kinetic measurements of GABA(rho1) currents were carried out in the presence or absence of antagonists. Use-dependent actions were also evaluated. 4 Picrotoxin, quercetin and pregnanolone exerted noncompetitive actions. 5 IC50 values measured at the EC50 for GABA (1 muM) were as follows: picrotoxin 0.6 +/- 0.1 muM (Hill coefficient n=1.0+/-0.2); quercetin 4.4+/-0.4 muM (n=1.5+/-0.2); pregnanolone 2.1+/-0.5 muM (n = 0.8 +/- 0.1). 6 These antagonists produced changes only in the slope of the linear current-voltage relationships, which was indicative of voltage-independent effects. 7 The effect of picrotoxin on GABA(rho1) currents was use-dependent, strongly relied on agonist concentration and showed a slow onset and offset. The mechanism was compatible with an allosteric inhibition and receptor activation was a prerequisite for antagonism. 8 The effect of quercetin was use-independent, showed relatively fast onset and offset, and resulted in a slowed time course of the GABA-evoked currents. 9 The effect of pregnanolone was use-independent, presented fast onset and a very slow washout, and did not affect current activation. 10 All the antagonists accelerated the time course of deactivation of the GABA(rho1) currents.